One other thing, but it's expensive.
Colt now offers the LE901 in .308 (7.62X51)
This lower will accept a Colt 5.56X45 AR-15 upper, so you can have both uppers on one lower.
It's a solution to the problem, but it is mighty expensive, and AFAIK, it's only available as complete units, bot parts.
(I might be mistaken on that one, but that is my understanding)
I didn't fight to defend your freedom so you could vote to take away mine.
"A man who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself.
John Stuart Mill."